Any ideas on what would cause this notch in my Hope Pro 4 freehub?

Last sprocket on the cassette not lined up right? Looks suspiciously square and any the right width for the narrow spline. Also there’s not much damage to the opposite side of the splines at that point compared to the rest of the length suggesting the outer sprocket was somehow tighter than the rest.

Last sprocket put on in the wrong place then tightened down / wore down over time?

edit – 5 seconds too slow! 5 seconds

@igm don’t worry.

Life is not a competition (but I’m winning)

are you sure thats not supposed to be there. Looks decidedly neat, with a radius corner and flat faces?
something to do with the last sprocket being quite different to all the others, being an end stop kind of thing?

I had a slightly loose cassette, removed & found exactly the same. Installation error. Filed down now OK.